Resonant reed motion detector

Claim 1

A MOTION DETECTOR COMPRISING, A FRAME, A MEMBER MOVABLE FROM A NEUTRAL POSITION RELATIVE TO SAID FRAME, A REED SUPPORTED AT ONE END ON SAID FRAME, AT LEAST THE FREE END PORTION OF SAID REED BEING MAGNETIC, MEANS COUPLED TO SAID MEMBER AND ARRANGED TO CONSTRAIN SAID REED INTERMEDIATE THE ENDS THEREOF ON MOVEMENT OF SAID MEMBER FROM THE NEUTRAL POSITION WHEREBY THE REED IS TUNED TO ONE RESONANT FREQUENCY, A SOURCE OF ALTERNATING CURRENT HAVING A FREQUENCY SUBSTANTIALLY HALF SAID ONE RESONANT FREQUENCY OF THE REED, EXCITATIONCOIL MEANS COUPLED TO SAID SOURCE OF ALTERNATING CURRENT AND ARRANGED ADJACENT THE FREE END OF SAID REED TO INDUCE OSCILLATION THEREOF AT SAID ONE RESONANT FREQUENCY, AND A MAGNETICALLY OPERATED CONTACT SWITCH RESPONSIVE TO SAID EXCITATION COIL MEANS AND DETECTING DETUNING OF SAID REED FROM SAID ONE RESONANT FREQUENCY.